In order to determine the roles of the apical ectodermal ridge and mesoblast in the production of limb anomalies, chick embryos at stages 18-20 of development will be injected with 6-aminonicotinamide, cytosine arabinoside, or saline. At stages 22-24, wing buds from control or treated embryos will be dissociated by trypsinization into ectodermal and mesodermal components and reassociated to form composite wing buds consisting of: a) a control apical ridge and a control mesoblast; b) a treated apical ridge and control mesoblast; c) a control apical ridge and a treated mesoblast; and, d) a treated apical ridge and a treated mesoblast. Composite limb buds will then be grafted to host embryos of the same stage to resume development. Twenty-four, 48, and 72 hours later, composite limbs will be obtained for light and electron microscopic examination to determine a) whether the mesoblast is necrotic or the matrix is reduced; b) the configuration of the apical ridge ectoderm, its basal lamina, and the adjacent non-ridge ectoderm; c) the ability of a treated mesoblast to maintain normal apical ectodermal ridge architecture; d) the appearance of the zone between the apical ectodermal ridge and mesoblast; and, e) the mitotic index in the subridge region of the mesenchyme. Scanning electron microscopy will be used to visualize large areas of the apical ridge in intact composite limb buds, and in composite limb buds which have been split longitudinally to examine the interface between the apical ectodermal ridge and mesoblast, and the height of the apical ridge ectoderm. Composite limbs will be sacrificed at 16 1/2 days of incubation, cleared and stained with alizarin red S to assess number of limb levels present and bone shape. Other hosts will be injected with India ink into the vitelline vessels to determine the vascular patterns of the composite limbs to relate the vascularity to the malformations. These studies will clarify the roles of the apical ectodermal ridge and mesoblast in the production of limb anomalies, and provide insight into the control mechanisms that regulate normal limb development.